The heating element, such as a bridge wire, of a conventional electric detonator is typically connected directly to the leg wires of the detonator. Its resistance can he measured by connecting the leg wires to a resistance meter (normally called a blasting ohmmeter) which uses much smaller electric currents than normal ohmmeters in the measuring process for safety reasons. However, in a transformer-based detonator, the bridge wire of the detonator is electrically isolated from the detonator's leg wires. Most often, the small transformer is encapsulated within the header of a detonator, leaving no direct electric access to the bridge wire. Therefore, it is not possible to measure the resistance of the bridge wire directly. That is, the resistance of the closed loop formed by the secondary winding and the bridge wire is isolated from DC in the primary. In use, the initiation energy is transformed from the primary winding to the secondary winding of the transformer via the magnetic linking between the two windings. Therefore, the resistance of the loop formed by the secondary winding and the bridge wire is designed to be in a certain range to receive the right amount of initiation energy so that the detonator can function reliably. If the loop resistance is too low, or too high, such as in the extreme cases of a short circuit or an open circuit, the detonator will fail to initiate. To make sure that the detonator receives the right amount of initiation energy, the bridge wire resistance is often designed to have a certain value with some tolerance. The measured loop resistance is the sum of the bridge wire resistance and the resistance of secondary winding. Since the resistance of the secondary winding is known and is determined by the design of the small transformer, the actual bridge wire resistance is obtained by subtracting the winding resistance from the loop resistance.
This apparatus described, and its use concern the method and circuitry for measuring the resistance of a closed loop, without touching the loop. In particular it can be used for checking the resistance of an initiation device which includes the use of two isolated windings. Examples of this kind of detonator are shown in U.S. Pat. No. 3,762,331 to Vlahos and U.S. Pat. No. 4,273,051 to Stratton. My co-pending U.S. patent application No. 08/992412 (assigned to Prime Perforating Systems) discloses the use of an isolated loop detonator in which a combination of different magnetic materials are used, enabling the detonator only to respond to a pre-determined frequency band. FIG. 1 is an illustration of such a detonator indicated generally as 20. A heating element, in the nature of a bridge wire 22 of detonator 20 has a resistance Rx. It forms a closed loop 24 with secondary winding 26. For safety considerations, loop 24 is electrically isolated from the primary winding, indicated as 30. The two windings 26 and 30, are magnetically coupled by a magnetic material 32 for the transmission of a firing signal having pre-determined characteristics, from leg wires 34. Since primary winding 30 is isolated from the bridge wire 22, it is not possible to measure the resistance of bridge wire 22 using a traditional blasting ohmmeter as is practised with conventional electrical detonators. However, the manufacturing process of the detonator requires that bridge wire 22 have a resistance as designed. Too great or too small a resistance may result in a detonator that does not explode as desired. Thus the measuring and monitoring of the actual bridge wire resistance is an important means of quality control. Also, a detonator is checked to assure that it is in good condition before it is used. There is a need for an instrument for measuring the resistance of such detonators. This instrument must not transmit a high energy test signal into the detonator, lest it explode. It must rely on a small signal and amplification.
U.S. Pat. No. 4,482,858 to Plichta discloses a method of testing a kind of electric detonator that has a ferrite core. The method is to use a small capacitor to store a calibrated amount of energy. This energy is discharged to a lead wire which forms the primary winding of the detonator. The value of the capacitor is calculated to ensure that the resulting RLC circuit, with the detonator to be tested, will be overdamped. Then the peak voltage over the primary lead wire is amplified and displayed. As the Plichta claims state, Plichta has "measurement means for effecting a single reading of a single peak value." It measures a peak value of a discrete pulse, then permits a relatively long time period to elapse. During this time period the pulse decays. After the first test pulse has died, a second test pulse is generated.
Since the parameters of the apparatus are determined by the inductance, and the resistance of the bridge wire, and since the apparatus is calibrated by the module of the detonator to be tested, it may tend to be suitable only for testing the detonator for which it is built. It would be advantageous to make a testing meter whose parameters are independent of the detonator to be tested, therefore, it should be flexible to test different designs of transformer-based detonators with minimal adjustments or calibrations.
U.S. Pat. No. 4,649,821 to Marshall describes an electrical circuit continuity test apparatus for testing high-energy-discharge circuitry of a firing unit. It includes the use of a transformer structure to sense the capacitance change in the secondary winding.